In the present article we consider the problem of underwater avalanches propagating over moderate slopes. The main goal of our work is to investigate the avalanche front velocity selection mechanism when it propagates downwards. In particular, we show that the front velocity does not depend univocally on the mass of sediments. This phenomenon is investigated and explained in our study. Moreover, we derive from the first principles a depth-averaged model. Then, we assume that sediments are uniformly distributed along the slope. In this case, they can be entrained into the flow head and a self-sustained regime can be established. One of the main findings of our study is that the avalanche front velocity is not unique due to a hysteresis phenomenon. We attempt to explain this phenomenon using dynamical systems considerations.